Recently picked up a used bike running a full 105 5600 groupset without much knowledge on chainrings and cassette sizes and the effect they have on how frustrated I get going uphill.

The current gearing is 53/39 and 11/25.

I have narrowed down my options to either picking up a new Ultegra 6800 groupset with compact 50/34 cranks and 11-28 cassette (because why not) or just replacing the cranks and cassette with some used ones for cheap.

5600 isn't a bad groupset, especially if the left shifter is the mid-model update 5601, which is less prone to breaking its guts. Its shift feel is considerably better than the 5700 that followed it. (SIDE NOTE: the original-release 5600 left lever was used for both double and triple chainring configurations, and needed exceptionally careful tuning on a double otherwise it could begin to shift into the third chainring position, jam and destroy its internals. In 2008 Shimano addressed this by introducing the double-specific 5601, and renumbering the original as 5603. The part numbers can be found by rolling the rubber hod forward from the handlebar and looking on the outer face of the shifter bracket. The front cover plates also give the clue; pre-update units only have the Shimano 105 logo, post-update also have the word "double" or "triple" as appropriate.)

The 5600 rear derailleur will handle up to a 28T rear sprocket. If you wish to go larger a 6701 Ultegra, 5701 or 4600 (but not 4700, it uses a different, incompatible cable pull ratio) Tiagra model will need to be fitted, which has revised geometry to allow it to fit up to 30T. It will work fine with the 5600 shifter.

You can fit a newer model Shimano crank without changing bottom bracket, but note that 2nd-generation 10-sp. (4600/4700/5700/6700/7900) and 11-sp. have the chainrings further apart to reduce noise, and need to be paired with a similar vintage front derailleur, which has a suitably greater swing off the same cable pull, in order to shift properly.

maybe the horse has bolted, but I would suggest the most cost-effective way to achieve this upgrade would be to purchase a 2nd hand bike with the desired group set in the first place. replacing your current gruppo will almost certainly cost you more in aggregate than just buying the right 2nd hand bike in the first place.

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